This invention relates to hermetically sealed electronic circuit modules.
Circuit modules have been hermetically sealed to protect the sensitive circuit components from harsh environmental conditions. Heretofore, it has been difficult to provide conductive paths between circuit components within the package to termination pads external of the package. Typically, chips or other circuit components are mounted in the package to one side of a wiring board or substrate and are electrically connected to termination pads within the hermetically sealed region. Conductive vias extend vertically through the board or substrate to connect the internal pads to pads outside the hermetically sealed region. The conductive vias are constructed by well-known plated-through or plated-up hole techniques by which holes in the board or substrate are plated or filled with conductive material to form the conductive vias. Alternatively, the vias may be formed by a deposition process. An example of the use of conductive vias to accomplish external connection in hermetically sealed modules may be found in U.S. Pat. No. 4,560,826 granted Dec. 24, 1985 to Burns et al.
The fabrication of the conductive vias require processing steps during the construction of the wiring board and/or substrate which are not normally incurred in processing the board. More particularly, conductive vias were employed in prior hermetically sealed modules only to extend through the board or substrate for external connection. Consequently, formation of these vias represented additional processing steps which added significantly to the cost of the module.
Another difficulty with hermetically sealed electronic packages and modules resides in the adequacy of the seal itself. Prior packages employed a metal ring and lid bonded by glass solder to a ceramic substrate. The glass insulated the ring from the wiring board to permit leads to pass thereunder from the region inside the sealed module to the external portions. One difficulty with such a metal-glass-ceramic construction is that environmental conditions, such as pressure and temperature variations, cause fatigue in the glass solder and failure of the seal. An example of the use of glass bonding of hermetic seals to packages may be found in U.S. Pat. No. 4,925,024 granted May 15, 1990 to Ellenberger et al.
To overcome the problems associated with metal seals, sealing rings constructed of a material more closely matching the thermal expansion characteristics of the ceramic substrate have been employed. For example, an iron-nickel-cobalt alloy marketed under the trademark KOVAR provides good thermal matching characteristics to the ceramic substrate. Also, KOVAR is provides good magnetic shielding. However, the iron-nickel-cobalt alloy is difficult to attach to ceramic substrates. Consequently, metalized ceramic sealing rings were used having KOVAR rings or flanges brazed thereto. An example of the use of KOVAR flanges brazed to ceramic sealing rings is found in U.S. Pat. No. 4,076,955 granted Feb. 28, 1978 to Gates, Jr.